In baseball, a pitcher is credited with a win (or "W") when, in a game won by his team, he is the team's pitcher at the time that his team takes a lead that it does not relinquish for the remainder of the game.

An exception to this is that a pitcher who starts a game cannot be credited with a win if he does not pitch at least five innings. If his team takes a lead it does not relinquish while that pitcher is in the game, the win is awarded to the relieving pitcher who, in the judgment of the scorer, pitched the most effectively. The winning pitcher cannot, however, be a pitcher who is credited with a save in the same game.

A loss (denoted by "L") is charged to the pitcher who, in a game lost by his team, is charged with allowing the run that gives the opposing team a lead they do not relinquish for the remainder of the game. The pitcher who gives up a hit to score the "go-ahead run" does not necessarily receive the loss, it goes to the pitcher who allowed the run-scoring player to reach base.

For example, in a tie ball game, if pitcher A walks a batter and then is relieved by pitcher B who then give up a home run, the first run allowed is recorded to pitcher A and the second to pitcher B. Thus, pitcher A gave up the run that gave the opposing team the lead, and pitcher A will be credited with the loss if the other team does not relinquish the lead, even though pitcher B was on the mound when the go-ahead hit occurred.

Every game (excluding the rare tie game) has both a winning and a losing pitcher. A pitcher's total wins and losses are commonly noted together; a pitching record of 15-10 indicates 15 wins and 10 losses.

A pitcher's winning percentage, standardly expressed to three digits, is calculated by dividing their wins by their total decisions (wins plus losses), as in the formula:

W / (W + L)

Records

Career wins

• Cy Young - 511
• Walter Johnson - 417
• Grover Cleveland Alexander - 373
• Christy Mathewson - 373
• Pud Galvin - 364
• Warren Spahn - 363
• Kid Nichols - 361
• Tim Keefe - 342
• Steve Carlton - 329
• John Clarkson - 328
• Roger Clemens - 328
• Eddie Plank - 326
• Nolan Ryan - 324
• Don Sutton - 324
• Phil Niekro - 318
• Gaylord Perry - 314
• Tom Seaver - 311
• Charles Radbourn - 309
• Mickey Welch - 307
• Greg Maddux - 305
• Lefty Grove - 300
• Early Wynn - 300

Career losses

• Cy Young - 315
• Pud Galvin - 302
• Nolan Ryan - 292
• Walter Johnson - 279
• Phil Niekro - 274
• Gaylord Perry - 265
• Don Sutton - 256
• Jack Powell - 253
• Eppa Rixey - 251
• Bert Blyleven - 250